Device for reducing the peak output of a pulse laser light source

The invention relates to a device for reducing the Peak power of a Pulsiaser light source, in particular for a Projektionsbelichtungsaniage.

Pulse laser light sources, for example excimer lasers, for the UV lithography have a repetition rate of about 1000 to 4000 pulses per second. The pulse length is currently 20 to 30 ns. Within each pulse occurs in dependence from the gaseous state, from the state of the laser, in particular the optical components, and depending on the cavity length to significant modulations of the laser output on the Time.

It has been found in practice that a significant Disadvantage of a pulse laser light source is that the short pulse duration causes a high pulse power density, to the optical materials, particularly glassy materials a very negative effect. Silica glass at a working wavelength of 193 nm is at a rate in which a power of the Power density received damaged. The consequences of this are Transmission losses and an uncontrolled increase in the Refractive index which limits the lifetime of the optical system. When SiO₂ in glassy form there are places with particularly weak Bonds. The pulse generated by the laser light source deliver pulsed shortwave electromagnetic radiations thus the energy to detrimentally these bodies modify.

From EP 0785473 A2 an apparatus is of the above-mentioned known type, through which the pulse from a laser light source split light coming into several partial beams will, through which bypasses. carried out in this way a bundle expansion or a division into several juxtaposed partial beams. These partial beams arranged side by side in a lighting device entered.

The disadvantage here is, however, that allow multiple optical axes next to each other produced and the lighting device adjusted accordingly is. Moreover, is the aforementioned Device fixed in their orientation.

In US 5,661,748 is a pulse delay device with partially transparent Mirrors and inevitable "tail" of the pulse distribution and an arrangement with two concave mirrors and a described gridded mirror in which, in principle, an offset (Part 4.2) occurs, not through the oblique passage created by Planplatten.

The present invention is therefore based on the object, to provide a device by means of which damage to Components housed in the beam path of the pulsed laser light source, be avoided if possible without any particular loss the efficiency of the pulsed laser light source.

According to the invention this object is achieved by the characterizing Part of claim 1 said measures dissolved.

According to the invention will now be the high points or peaks of mined pulse laser light source, and reduces the peak power density, namely with little integral energy penalty. is performed by the beam splitter in communication with the bypass line a division of the beam of the pulsed laser light source in Partial beams with an optical path difference. It is only to ensure that the path difference is selected so is that at least largely complementary peaks and valleys. Peaks and valleys are separated in time, for example 7.0 ns. This corresponds to a path of light of 2.1 m. to all halving pulses in their pulse power density, they are divided into a plurality of pulses and then lined up in time. To the greatest possible reduction of the peaks within a pulse and a substantially uniform power density to generate, in an advantageous embodiment the invention proposes at least two bypasses in Beam path behind the other to arrange.

According to the invention not several partial beams are produced which arranged side by side with a corresponding number of optical axes, but it carried a reunion the partial beams to a total beam, whereby a uniform optical axis for subsequent illumination device given is. It takes place after the division into several partial beams Finally again a Auffädelung instead.

In a particularly advantageous manner can the invention use device for influencing of polarized light, what accordingly between the beam path and the Beam splitter means an angle, preferably Brewster's angle, is provided. The Brewster angle is reached, that the beam splitter device, such as a mirror, pass 50% of the beam unimpeded and without losses (Apart from absorption in the splitter layer), while the other 50% are derived. Of course, in the Scope of the invention but also other angles and thus also other division percentages possible.

In a further embodiment of the invention, provided be that the beam recombining member is so designed that part of the partial beam, which run on the detour line is sent repeatedly through the bypass line.

In this way, even a plurality of optical path differences and thus achieve an even greater homogenization. Of course, additional losses from mirrors come and divider for carrying.

Through a phase plate in the beam path is obtained influencing the power peaks and the life of the optical materials subjected to the pulse laser light source are.

Further advantageous refinements and developments from the remaining dependent claims and from the following principle with the embodiment of the drawing.

It shows:<dl tsize="8"><dt>figure 1</dt><dd>a schematic diagram of the beam path of a Pulse laser light source with two bypasses;</dd><dt>figure 2</dt><dd>the power of the pulse laser light source in the division state by a first bypass line;</dd><dt>figure 3</dt><dd>the performance history of the pulse laser light source according to a second bypass line;</dd><dt>figure 4</dt><dd>a further embodiment of the invention Device in a schematic representation; and</dd><dt>figure 5</dt><dd>a schematic diagram of a projection exposure system with an excimer laser and the inventive Device.</dd></dl>

As shown in Figure 1 applies a beam 1 a pulse laser light source such as an excimer laser 2 (Shown in Figure 5) to a first beam splitter means in the form of a mirror 3. 50% of the total beam pass unhindered as partial beam 1a and the lossless Beam splitter 3 toward a second beam splitter 4 as the second beam splitter. The other 50% of the beam make their way as a partial beam 1b via a bypass line 5, the three levels 6, 7 and 8 as reflective components is formed. The beam splitter 3 as a first beam splitter acting on its reverse side, at the same time as Beam combining element 9, in which the partial beam with the ib passing through the beam splitter means partial beam la is reunited. Through the first detour line 5 is the Pulse of the pulse laser light source 2 is smoothed in itself. simultaneously is this the peak power by 30 to 40% reduced. Due to the angular position of the mirror 3 and the first divider Detour line 5 to the vibration direction of the incident linearly polarized laser light passes, the amount of polarization degree the energy balance is not important. Both partial beams 1a and 1b are polarized in the same at the output of the first detour line 5, but at different times.

From Figure 2 is the time offset of the two partial beams 1a and 1b recognized. As can be seen, therefore, the peak power of the pulse laser light source, which due to the beam-splitting each has been reduced to half, at a Addition due to the time offset of the two "peaks" P1 and P2 in accordance with the resulting total peak reduced in comparison with an untreated beam path.

The laser light beam after the reunification of the two Partial beams 1a and 1b in the further beam path by the second Beam splitting means in the form of the splitter mirror 4, the turn 450 rotated (Brewster angle) to the direction of oscillation the laser light is, in the partial beam 10a of the beam splitter 4 freely pass, and the sub-beam 10b is sent via a second bypass line 11, divided. The second bypass line 11 is also by mirrors 12, 13 and 14 are formed as reflective components. The back of the Beam splitter 4 again serves as a second beam combining member 15, by which the two partial beams 10a and 10b again composed with the same polarization direction and then be sent together.

Through the second bypass line 11, for example, a path of travel of may have 11.1 m, the pulse of the pulse laser light source 2 is added as a whole. In this way, at the output or at the beam combining element 15, two pulses with the same polarization, which are largely smoothed or not inadmissible high "peak" longer have that to damage of components could result.

From the figure 3, the power versus time, and the two Pulse seen as after the beam combining member 15 present. The original power peak can on about one third of the original value can be reduced. For example, when the temporal pulse duration of the pulse laser light source 30 ns, this corresponds to a path of light of 9 m. If you combine according to the embodiment, the first Detour line 5 with the second bypass line 11, so once 2.1m delayed and smoothed in this way Pulse 9.0 m + 2.1 m, for a total of 11.1 m delayed in the second bypass line 11th

For the inventive measures are most suitable areas between the laser output and a scanner input.

The particular advantage of the invention lies in the fact that the subsequent illumination unit are not specifically designed must, but the proposed establishment only between Laser output and input of the lighting part of a Scanner is inserted. This is also an economic Retrofitting of existing systems (no new lighting part the scanner).

When the beam quality of the pulse laser light source by the different path lengths suffering, this example can be prepared by a Kepler telescope (in individual cases to the excimer laser divergence slightly adapted), with its two lenses 16a and 16b - as in the Figure 1 indicated - produced in the second bypass line 11 again will. Also, a single lens is eligible.

Of course, it is also possible for the above- to extend said structure and as yet more phasing lines provide, if deemed physically impractical and a further lowering of excellence still benefits brings.

Likewise, it is also conceivable to divide the beam physico instead of optical polarization. If the peak power of a pulsed laser light source for example by a corresponding time offset the sub-beams is reduced by half, thereby reducing the risk of damage to components not only by half, but even more. As the "peaks" in the single pulse are structured by which the resonator length comes in the laser, can be already with short delay paths achieve that the pulse added in the course of its length and the structuring is utilized in accordance with which a significant flattening can be achieved. Due to the structure the pulses of pulse laser light sources can be here clearly even by small bypasses the power peaks to reduce. An optimum is achieved, of course, if the postponement by the beam splitting so is great that allow each "peak" and valleys assign. This can be, for example through the second bypass line 11 reach. Instead of the mirror as reflective components can also other reflective components, such as prisms provided will.

one generated by a λ / 4 plate (not shown) prior to the Means circularly polarized light is any azimuthal Angle about the Z axis for the beam splitting means 3 and 4 possible.

In the figure 4 is a further embodiment of the invention Device shown in principle. As can be seen, is performed a division of the beam 1 to the beam splitter means 3 in a partial beam 1a through the beam splitter means 3 passes, and a partial beam 1b the way through the rectangular bypass line 5 takes. The detour line 5 is this case by four mirrors 17, 18, 19 and 20 formed before the beam part 1b on the rear side of the beam splitter means 3, the same as in the embodiment used by the figure 1 as a beam combining member 9, again is combined with the partial beam 1a.

takes place in contrast to the embodiment of the figure 1 while not a complete union, but the embodiment of Figure 4 is designed such that a portion of the way past on the bypass line 5 part-beam 1b once more the way of the bypass line 5 takes, wherein This can be performed multiple times. Coupling of of the beam takes place in this embodiment in the manner of a resonator, wherein the proportion of the detour via conduit 5 revolving light or partial beam 1b, ie, the adjustable Pulse length, depending on the state of polarization. To this Example, any delays of the incoming pulses and an associated reduction of the peak pulse power of Pulse laser light source are achieved. It is only for to ensure that an optimum balance between extending the time frames the partial pulses (number of revolutions resonator) and the total losses chosen is. This is achieved by the arrangement a phase plate 21 in the beam path before Spotlights splitter 3 and a further phase retardation plate 22 in the bypass line 1b. In this way, from a linearly polarized light, a slightly elliptically polarized made light which in passing through the beam splitter means 3 are reflected by a corresponding component becomes. The height of the component depends on the degree of Ellipse from. you Equips as the phase retardation plate 21 with a λ / 4 value, then an amount of 50% is the detour line 1b sent. In practice, one λ-values between 0 and select a quarter.

The embodiment according to Figure 4 with the two phase retardation plates 21 and 22 has the advantage that in this way the device can be extremely versatile. Substituted it namely one or both phase retardation plates 21 and 22 so changes in accordance with the peak power. Would like to achieve as a higher power, which forced a higher load and thus a shorter lifetime for the Apparatus arises, we will phase retardation plates 21 and 22 are running, the force correspondingly fewer rounds. Conversely, one can in this way by a corresponding Increasing the number of rounds of light a conservation reach of the optical components of the projection exposure system.

In the figure 5 is in principle an application point of invention Device 22 shown. The polarized Light source 2 is then for example a projection exposure system provided that in one of the extending through a wall 17 of Near separate clean room with an illumination optics 18, a mask 19, a projection lens 20 and a laser 21 is. In order to influence the clean room as little as possible can one case, the device 22, the inside of the in figure 1 Parts shown is provided between the pulse laser light source 2 and the wall 17 in the beam path of one of the pulse laser light source 2 Arrange.